1. Field of the Invention
The present invention relates to a battery charger for charging secondary batteries, such as nickel-cadmium batteries and nickel-hydrogen batteries.
2. Description of the Related Art
Battery packs accommodating therein a nickel-cadmium battery or a nickel-hydrogen battery are used as power sources for a various kinds of portable devices, such as a cordless power tool. The battery pack for the cordless power tool typically accommodates a temperature sensor along with the battery. Battery temperatures detected by the temperature sensor are used to determine the full charge condition of the battery.
Japanese Patent Application Publication No. HEI-6-113475 discloses a dT/dt detection method for determining the full charge condition using the detected battery temperatures. Specifically, while detecting the battery temperature at every sampling time with the use of the temperature sensor, a battery temperature gradient (hereinafter abbreviated to “BTG”) is computed each time the battery temperature is detected based on the newly and previously detected battery temperatures. The battery is determined to be full charge when the currently computed BTG value is larger by a predetermined value than the minimum BTG value.
The battery packs are removed from the portable device when the battery runs low, charged with a battery charger, and subsequently reloaded into the portable device. The series of operations as described above are repeated multiple times as far as the cycle service life of the battery pack permits. However, the repetitive loading of the battery pack into and removal from the portable device abrades the contacts of battery pack, or else the contacts are corroded as a result of secular change. Abrasion or corrosion of the contacts increases contact resistance. The same is true with respect to the terminals of battery charger, which are in electrical connection with the contacts of battery pack.
The battery pack has positive and negative terminals to be connected to the battery terminals. Also the battery pack has a temperature measurement terminal to be connected to a temperature sensing terminal at the side of the battery charger. A voltage developed across the temperature sensor is applied to the battery charger through the contact of the temperature measurement terminal and the temperature sensing terminal. Amongst the three terminals of the battery pack, the negative terminal is used as a common terminal of the temperature sensor and the battery. When the battery pack is charged while connecting its positive and negative terminals to the corresponding battery terminals, a contact voltage Vc is induced due to contact resistance existing in a contact area of the two terminals. The voltage on the battery side terminal is higher than the voltage on the charger side terminal. The output voltage from the temperature sensing terminal is higher than a real value due to the contact resistance Vc.
Switching the charge current during the charging operation changes the contact voltage, which in turn changes the output voltage from the temperature sensor. Accordingly, switching the charge current results in a virtual big temperature change. Hence, the computed BTG value may erroneously indicates that the battery pack has reached a full charge condition.